You’re recieving this error because WordPress notices the post_type query argument and that sampletype is a valid registered post type, and thinks you’re on an edit page. Try using a different query arg then post_type.
Note that admin.php?page=plugin-slug-posttypes-add&post_type=thisisnotaposttype and admin.php?page=plugin-slug-posttypes-add&foo=bar will both work fine.
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